The manufacture of integrated circuits generally involves using compact projection aligners, such as steppers, that focus a high-intensity light beam through optical elements (e.g., lenses) to transfer a circuit pattern onto a semiconductor wafer.
There has been a growing demand for compact projection aligners with increased resolution. One method for increasing resolution is by shortening the wavelength of a light source used in the compact projection aligner. For example, a stepper may include a high-output excimer laser that uses a mercury lamp as the light source for generating light in the short-wavelength range. Examples of excimer lasers that generate a short wavelength light beam include KrF excimer lasers (.lambda.=248.4 nm) and ArF excimer lasers (.lambda.=193.4 nm).
An anti-reflective coating is formed on the optical elements in the stepper in order to decrease light loss, flare, and ghosts, etc., that result from surface reflections. Anti-reflective coatings, however, may also cause loss of light due to absorption. The degree of absorption may be particularly high when a coating is exposed to short-wavelength light, such as excimer laser light. Additionally, heat generated by absorption can break down the anti-reflective coating and deform the substrate surface. For the aforementioned lasers, the coating materials having the lowest absorbency and the highest degree of laser resistance (i.e., durability to exposure of high-intensity UV light, especially UV laser light, such as excimer laser light) are limited to fluoride compounds, such as magnesium fluoride (MgF.sub.2), and some oxides. The optical substrates with which such coatings are used are limited to fluorite crystals, silica, and quartz.
A known anti-reflective coating is shown in FIG. 8. The coating has a 2-layer composition including a first layer 13 with a high refractive index and a second layer 14 with a low refractive index laminated onto an optical substrate 11. The desired anti-reflection conditions of the coating are defined by the following equation: EQU n.sub.s /n.sub.o .ltoreq.(n.sub.2 /n.sub.1).sup.2 ( 1)
The term n.sub.s denotes the refractive index of the optical substrate; the term n.sub.o denotes the refractive index of the medium through which the light travels (i.e., typically air having a refractive index of 1); the term n.sub.1 denotes the refractive index of the second layer 14 (i.e., the low refractive index layer), and the term n.sub.2 denotes the refractive index of the first layer 13 (i.e., the high refractive index layer).
When applied to a known coating using a high intensity light source with a design center wavelength of .lambda..sub.0 =193.4 nm, equation (1) is not satisfied. For example, if the optical substrate 11 is made of quartz glass (n=1.56), the high-refractive index layer 13 is lanthanum(III) fluoride (LaF.sub.3, n=1.69), and the low-refractive index layer 14 is magnesium fluoride (MgF.sub.2, n=1.42), the result is as follows: EQU 1.56/1&gt;(1.69/1.42).sup.2 .apprxeq.1.42 (2)
Notably, the equation is not satisfied because the left-hand side of the equation is greater than (rather than less than) the right-hand side. Thus, this embodiment is insufficient to meet the anti-reflection conditions.
For the FIG. 8 embodiment, the optical substrate and coating materials used and the associated thicknesses of each are as follows:
______________________________________ Substrate: quartz glass First layer: LaF.sub.3 (0.25).lambda..sub.0 Second layer: MgF.sub.2 (0.25).lambda..sub.0 Medium: Air ______________________________________
FIGS. 9, 10, and 11 show the characteristics of a 2-layer conventional anti-reflective coating. FIG. 9 shows the reflectance as a function of wavelength with an angle of incidence .theta.=0. At .lambda.=193.4 nm, the 2-layer anti-reflective coating has a residual reflection of about 0.2%. When used in an optical system having 50 lenses, light passes through one hundred anti-reflective coating surfaces. Due to the resulting residual reflection, close to 20% (1-(1-0.002).sup.100 =0.18) of the light does not pass through the optical system. This translates into a substantial decline in exposure efficiency. Furthermore, the significant reflection also can cause flare and ghost, etc., which reduces the exposure precision.
FIG. 10 shows the reflectance as a function of angle. The reflectance is at about 0.2% with .theta.=0.degree. and is greater than 0.5% at approximately .theta.=32.degree..
FIG. 11 shows the optical admittance when .lambda.=193.4 nm. Preferably, the end point of the admittance locus should come close to the point (1,0) expressing the refractive index of the medium, air. FIG. 11, however, shows the optical admittance is skewed to the right, which further illustrates the deficiency of the 2-layer coating.
In order to meet anti-reflection requirements, an optical substrate having a lower refractive index with a high-refractive index material and a low-refractive index material laminated on the optical substrate may be used. However, because the types of optical substrates and coating materials used in the 2-layered approach is limited, as described above, residual reflections still occur.